Mass transfer critical surface tension

Supercritical fluid extraction (SFE) utilizes the properties of supercritical fluids for extraction of analytes from solid samples. A supercritical fluid (SCF) is a substance above its critical temperature and pressure, when it is between the typical gas and liquid state. Low viscosity and near-zero surface tension and heat of vaporization allow SCFs to penetrate into solids more rapidly than liquid solvents, which leads to more favorable mass transfer. The density of an SCF is close to the liquid density. 

Case e is the result of precipitation under conditions above the ternary MCP and is the true supercritical antisolvent precipitation. The formation of very small particles indicates that characteristic times for surface tension disappearance are very short, droplets are not formed at the exit of the injector and the very small particles are released from the fluid phase. These aspects have been studied in the previously cited works of Werling and Debenedetti," which underline the interplay among the different consecutive or simultaneous process that characterize SAS, mainly mass transfer under sub-critical and supercritical conditions. The characteristic times... 

A very important aspect of phase behavior in a system consisting of a volatile organic solvent, such as ethanol, and a supercritical fluid, such as CO2, is that the mixture critical pressure coincides with the liquid vapor phase transition. This means that above a single phase exists for all solvent compositions, whereas the (ethanol-rich and C02-rich) two-phase region lies below this curve. This fact has important implications for the mass transfer and precipitation mechanisms. Complete miscibility of fluids above P means that there is no defined or stable vapor liquid or liquid liquid interface, and the surface tension is reduced to zero and then thermodynamically becomes... 

CO2 offers many advantages over conventional solvents because it is non-toxic, nonflammable, inexpensive and readily available. Moreover, CO2 presents a relatively low critical temperature (31.1 °C) and moderate critical pressure (7.4 MPa) and as with all supercritical fluids (scFs), it offers mass transfer advantages due to its low viscosity and surface tension. 

Accordingly, a hypothesis was formulated, which seeks the theoretical rationale of appearance of the specific flow structures inside the liquid layer. It is known, that under certain circumstances, the surface tension variations may lead to the flow instability and to the induction of convection cells [e.g., 5, 6]. Our preliminary theoretical analysis of the hydrodynamic stability of the system [7] indicated that it is possible, that for the Reynolds numbers exceeding the critical value, convection cells inside the hypophase can be formed. This should lead to the significant increase of the mass transfer rate. 

In order to detect species which are produced at an electrode surface by mass spectrometry, they have to be transferred from the electrolyte phase to vacuum. Fortunately, for aqueous systems [2] and also for some organic electrolytes with a high surface tension, e.g., propylene carbonate [7], the separation of the electrolyte from the vacuum can be achieved by using porous Teflon membranes. Due to their hydrophobicity, the liquid does not penetrate into the pores, whereas dissolved gaseous and other volatile species readily evaporate in them. The critical pore size depends on the... 

In the course of interfacial mass transfer, from molecular point of view, the process is stochastic, that means some local molecules may undergo the mass transfer in advance than the others, so that small concentration gradient (where i — x, y, z) is established at the interface. As the surface tension a is function of concentration, it follows that the surface tension gradient is also created at the interface. If is increased up to a critical point, the fluid dynamic instability will appear to induce the interfacial convection as well as the formation of orderly structure at the interface. At the same time, the rate of mass transfer may be enhanced or suppressed depending on the properties of the mass transfer system concerned such phenomena is generally regarded as interfacial effect. 

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